U.S. patent application number 11/009015 was filed with the patent office on 2006-01-26 for dual power bus for battery powered device.
Invention is credited to Peter J. Frith, David Sinai.
Application Number | 20060017423 11/009015 |
Document ID | / |
Family ID | 32922806 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060017423 |
Kind Code |
A1 |
Frith; Peter J. ; et
al. |
January 26, 2006 |
Dual power bus for battery powered device
Abstract
The present invention relates to battery power peripheral
devices such as MP3 players which are also periodically connected
to another power source such as a mains wall socket or USB cable
power bus. In particular, but not exclusively, the present
invention relates to regulation of these voltage sources. In
general terms the present invention provides a dual supply rail for
the load regulators of a power supply circuit for a battery powered
device. One supply rail is coupled to the battery, and the other is
coupled to a non-battery source such as an external mains regulated
source and/or a bus power wire from a USB cable or similar. The
regulators have dual inputs, each for taking their input voltage
from one of these supply rails.
Inventors: |
Frith; Peter J.; (Edinburgh,
GB) ; Sinai; David; (Edinburgh, GB) |
Correspondence
Address: |
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
2101 L Street, NW
Washington
DC
20037
US
|
Family ID: |
32922806 |
Appl. No.: |
11/009015 |
Filed: |
December 13, 2004 |
Current U.S.
Class: |
323/268 |
Current CPC
Class: |
H02J 7/0068 20130101;
H02M 1/10 20130101; H02J 7/34 20130101 |
Class at
Publication: |
323/268 |
International
Class: |
G05F 1/56 20060101
G05F001/56 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 26, 2004 |
GB |
0416631.0 |
Claims
1. A power supply circuit for a battery powered device, the circuit
comprising: an input for receiving a non-battery voltage supply and
coupled to a first power supply bus; an input for receiving a
battery voltage supply and connected to a second power supply bus;
a common dual input regulator for providing a regulated power
supply to said battery powered device, the regulator having two
input pass devices, one said pass device connected directly to the
first power bus and the other said pass device connected directly
to the second power bus, the common regulator arranged to derive
the regulated power supply from the first power bus or to derive
the regulated power supply from the second power bus; and a
charging circuit for charging the battery supply and coupled
between the first and the second power buses.
2. A circuit according to claim 1 wherein the pass devices are
MOS-based transistors.
3. A circuit according to claim 1 wherein the common regulator
comprises switches in order to switch one or the other said pass
devices into a regulation sub-circuit in order to derive the
regulated power from the first power bus voltage supply or the
second power bus voltage supply accordingly.
4. A circuit according to claim 1 wherein the pass device connected
directly to the first power bus has a different on-resistance from
the other pass device connected directly to the second power
bus.
5. A circuit according to claim 1 wherein the common dual input
regulator is one of: a linear regulator; a switch mode regulator, a
capacitor charge pump regulator.
6. A circuit according to claim 1 further comprising an input for
receiving a second non-battery voltage supply and coupled to the
first power supply bus.
7. A circuit according to claim 1 wherein the first non-battery
voltage supply is a composite power and data cable connection.
8. A circuit according to claim 7 wherein the cable is a USB or
EEEE1394 cable.
9. A circuit according to claim 7 further comprising a DC-DC
converter coupled between the battery supply voltage input and the
first non-battery supply voltage input.
10. A circuit according to claim 1 further comprising an input
regulator coupled between the first non-battery input and the first
power supply bus.
11. A dual input regulator for a power supply circuit, the
regulator comprising: two input pass devices, one said pass device
for connecting directly to a first power bus and the other said
pass device connected directly to a second power bus, the regulator
arranged to derive the regulated power from the first power bus or
to derive the regulated power from the second power bus; wherein
the two pass devices are MOS-based transistors.
12. A regulator according to claim 11 wherein the two input
transistors each have first connections connected respectively to
the two voltage supply buses; the two input transistors each having
second connections connected to a regulated output, the output
being connected to an input of an error amplifier, the other input
of the error amplifier being connected to a reference voltage; the
two input transistors each having third connections switchably
connected to the output of the error amplifier.
13. A regulator according to claim 11 wherein the pass device
connected directly to the first power bus has a different
on-resistance from the other pass device connected directly to the
second power bus.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to battery powered peripheral
devices such as MP3 players or cell-phones which are also
periodically connected to another power source such as a mains wall
socket or USB cable power bus. In particular, but not exclusively,
the present invention relates to regulation of these voltage
sources.
BACKGROUND OF THE INVENTION
[0002] FIG. 1a shows a power supply system for a typical
battery-powered peripheral device. The system comprises connections
11 and 13 to external power supplies, as well as to a re-chargeable
battery 10. The external supplies are typically regulated (12)
and/or switched to a common external supply node (Vsup) which
supplies charging current to the battery through a charging circuit
or controller 14. The battery 10 is coupled to an internal power
bus which supplies one or more load regulators 15 which provide
regulated voltage outputs to one or more parts of the peripheral
device. For example one load regulator 15a may supply the disk
drive of an MP3 player, whilst another regulator 15b supplies its
signal processing and amplifier circuitry.
[0003] The battery 10 can be recharged from the power wires 11 in a
bus cable, such as a USB or IEEE 1394 connection. Supply current
taken from the bus power wires 11 will generally first pass through
a supply bus regulation block 12. In the case of USB, this is
required to guarantee that the current taken from the bus be
limited to 100 mA or 500 mA. For the case of 1394 the supply
regulator 12 is required to attenuate a possible 48V to the 5V or
so maximum that the power supply system circuitry can tolerate.
Techniques for such regulators, generally involving sensing the
output voltage and current and feeding these signals into one or
more feedback loops are well known to those versed in the art.
[0004] In an alternate mode of operation, some or all of the
required supply current can be sourced externally (13) via a
transformer (not shown) attached to the mains, or perhaps from a
12V nominal source from a car battery. This supply voltage is again
generally pre-regulated to 5V or so, for example by a linear or
switching regulator (not shown). There may also be a means for
selection between the bus (11) and the external (13) supplies, for
example either diodes in series with one or more of these supplies
or more intelligent control with comparators and controlled
switches. For simplicity these are not shown in FIG. 1. These
non-battery supplies (11, 13) are coupled to a common node or
voltage rail Vsup, which is then supplied by one of these external
supply sources (11 or 13).
[0005] Current supplied to the battery 10 must be regulated in
current to limit the current when charging, and in voltage to
prevent over-charging of the battery. This function is achieved
using a charging control block or circuit 14. For example a Li-ion
battery will typically be charged at constant current (typically
0.5 to 1.0 C amperes, where C is the battery capacity in
ampere-hours, say 800 mAh) until its terminal voltage reaches 4.2V,
then it will be charged at constant voltage of 4.2V until the
current taken drops to near zero. Techniques for such charger
regulators 14, generally involving sensing the output voltage and
current and feeding these signals into one or more feedback loops,
are well known to those versed in the art.
[0006] Depending on the state of charge of the battery 10, its
output voltage Vbat can vary from say 4.2V when fully charged to as
low as about 2.7V before the battery becomes so discharged as to
cause irreversible degradation in its capacity. As high-current
loads (e.g. motors) are switched on and off, this battery terminal
voltage may also vary due to the output impedance of the battery
(say 100 m.OMEGA.). Some circuitry may be able to accept this
unregulated voltage direct from the battery. This may be attractive
in terms of system cost. However most circuitry will require a
cleaner, better regulated, supply, perhaps regulated at 1.8V or
3.3V for logic circuitry, and higher voltages for other
applications, such as 7.2V for driving banks of white LEDs for
example. There will thus generally be one or more voltage
regulators 15a and 15b driven from the battery line Vbat.
[0007] Depending on the input and output voltage levels, the
efficiency required, and the cleanliness required of the supply,
these voltage regulators 15 may be capacitive charge pumps, or
inductive buck or boost switching regulators, or linear regulators.
These are shown in simplified from in FIG. 1a, and in more detail
in FIG. 2. FIG. 2a shows a low-drop-out linear regulator; FIG. 2b
shows a non-low-dropout linear regulator; FIG. 2c shows a buck
switching regulator; FIG. 2d shows a buck-boost switching
regulator; FIG. 2e shows a boost switching regulator, FIG. 2f shows
a non-inverting boosting capacitor charge pump regulator, and FIG.
2g shows an inverting capacitor charge pump regulator. Many other
well-known variants of regulator exist, including those where
diodes are replaced by appropriately switched pass transistors.
[0008] Except for the simple boost regulator (FIG. 2e), all these
circuits contain a switch-type input pass device Mp connected
directly to the input supply, and to a regulator internal or output
node Vx. The charge pump of FIG. 2f includes two such devices. The
boost regulator of FIG. 2e includes an input inductor rather than a
switch-type device and which is connected to a similar internal
node Vx.
[0009] Note that the various pass devices are shown as MOSFETs but
may be any suitable device including NMOS, PMOS, diodes, or bipolar
transistors or even relays where suitable.
[0010] Generally, if the alternate supply (13) is available, it
will be used in preference to the bus supply (11) or the battery
(10). If no alternate supply is available, the bus supply will be
used if possible. Only if neither the bus supply nor the alternate
supply is available will the battery supply (10) be used. This
operation can be realised for example by sensing the voltage on the
various supplies and controlling various switches depending on
which of these supplies exceed respective thresholds. Such control
techniques are well known to those skilled in the art.
[0011] An example of a similar type of power supply is disclosed in
Maxim Integrated Products' data sheet reference MAX1874. As shown
in FIG 1b, this merges transistors or pass devices Ma and Mb and
their controls 12 and 14 from FIG. 1a, and couples the alternate
supply to the battery via a parallel transistor Mb2 and control
14'. This chip does not include the downstream regulators, but they
would typically be connected to the battery as shown, to allow the
system to function powered from the battery in the absence of the
supplies.
[0012] One problem with this type of scheme relates to the time
which the system takes to become active when powered from the bus
(11) or alternate (13) supply with a discharged battery 10. The
load regulators 15 will have a minimum input voltage, perhaps 3.2V,
(or maybe as high as 3.6V for a 3.3V linear low-dropout
regulator--FIG. 2a), whereas the battery may initially be
discharged below this voltage. Thus the system supplied by the load
regulators 15 will not work properly until the battery 10 is
charged up. Where the battery is heavily discharged, this might
take several minutes or longer. If the battery is deeply
discharged, below say 2.5V, the battery charging current is in fact
typically reduced by a factor of ten in order to minimize battery
capacity degradation effects and also as a safety mechanism due to
the fact that in the absence of adequate power there may be no
software control of the system. In this situation the wake-up time
will clearly be even longer. This behaviour is undesirable to
consumers who now desire "instant-on" behaviour.
[0013] A further problem is that the charger current control 14 or
14' limits the current to the node Vbat, to avoid too rapid
charging of the battery 10. However it cannot differentiate between
current taken by the battery 10 and that taken by the other loads,
for example the regulators 15. Thus if the battery charging current
is limited to 100 mA, then the total taken by the loads is also
limited to 100 mA. Thus if they take 99 mA, only 1 mA is available
to charge the battery, further increasing the time required for the
system to operate properly. Even if the error is less gross than
this, and say there is only a 25% reduction in charger current
actually reaching the battery, this may well confuse the analog or
digital control of the battery charging process, affecting the
effective Icharge-Vbat trajectory, and causing a charging time that
is still sub-optimum, even allowing for the 25% reduction in
battery charging current.
[0014] In this kind of scheme the system current is also limited to
the maximum current allowed by the charger 14 or 14', which means
that whenever the overall system current (including regulator input
current) requires a higher current than allowed by charger control,
this current would be drained out of the battery 10. This is not
just extending charging time it is also decreasing the battery life
time.
[0015] The circuit could be improved by sensing current flowing
only into the battery 10, while controlling all current into Vbat,
but this still does not guarantee adequate current into the other
loads 15, as the splitting of current will be defined by the
respective V-I characteristics of all circuits connected to the
Vbat node, including the battery, so a discharged battery would
tend to steal current away from loads expecting a higher voltage.
This means that this current would be taken by the battery 10 as a
priority, rather than by the loads 15 as a priority.
[0016] FIG. 3 illustrates one solution to this problem of the
"instant-on" requirement. The load regulators 15 are now supplied
from the bus supply and external supply common node Vsup, rather
than directly from the battery 10. As the battery node Vbat can be
isolated (switch Mc and charging control 14) from this common
supply node Vsup, the system can wake up as soon as power is
applied either from the bus (11) or the alternate (13) supply. Only
when neither the alternate supply (13) nor the bus (11) can supply
current, an additional battery switch Mc is turned on, and the load
regulators 15 are then supplied from the battery 10.
[0017] Examples of similar arrangements are disclosed in Linear
Technology Corporation's data sheet references LTC3455 and
LTC4055.
[0018] In these cases the battery charger supplies only the
battery, so the charging current can be accurately monitored to
allow intelligent control of the charging current-voltage
trajectory.
[0019] Also when driving the system from the bus or alternate
supply, this arrangement avoids the power losses associated with
passing current through the charger regulator prior to being input
to following switching regulators. Efficiency per se may not be a
major concern when driving from non-battery supplies, but reducing
power dissipated may allow less heat-sinking and hence lower system
cost.
[0020] The main problem with this solution is the extra voltage
drop between Vbat and Vsup when the load regulators 15 are driven
from the battery 10 compared to the system of FIG. 1. The battery
voltage is at best 4.2V, and should work down to as low as possible
to extend operating time between battery recharging (albeit
avoiding deep discharge, below about 2.6V). The load regulators 15
require a minimum input voltage (regulated output voltage plus
dropout voltage) in order to maintain regulation of their output
voltage, so the regulators will continue to function correctly
until the battery discharges to this minimum input voltage. However
the voltage drop across this additional switch device (Mc)
effectively increases the minimum voltage required from the
battery, and hence reduces the time the battery can provide this.
The voltage drops across switch devices (Ma, Mb, Mc) increases with
their on-resistance.
[0021] Given the technologies available today, these switch devices
will generally be implemented using MOS switches, rather than
bipolar transistors or relays. Lower on-resistance discrete MOS
switches are more expensive as they require larger silicon area or
more complex and specialized wafer processing. More particularly,
for systems where most of the circuitry of FIG. 1 or 3 is
implemented on a single chip, the total area required for these
switches has not only an impact on chip area and hence cost, but
also may require so much area that the silicon die may not fit in
the desired plastic package. This is especially critical for
portable equipment such as MP3 players or mobile phones, where the
size of the whole system is an important specification and requires
the smallest possible package size.
[0022] To allow Vbat to reach 4.2V when fed from a 4.5V bus supply
(11), Ma and Mb might be sized to drop 150 mV each at peak battery
charging current. But the sizes of input transistors Mp in the
switching or linear load regulators 15 will define a minimum input
voltage to keep their respective outputs in regulation. So either a
substantial reduction in operating battery life has to be
tolerated, or the input switches Mp of the load regulators 15 have
to be greatly enlarged and possibly even extra bond wires and
package terminals added as the parasitic resistances involved in
tracking the current from chip to the outside world are
significant. For example if a minimum battery voltage of 3.6V has
to supply a 3.3V output linear dropout regulator 15b, then battery
switch Mc and the regulator's pass device Mp have to be designed
for a 150 mV drop-out voltage each at peak load current.
[0023] There is also a possible issue of problems arising from
modulation of the voltage on Vsup caused by load variations on the
load regulators 15. As downstream peripherals are plugged in, or as
a disc drive internal to the battery-powered peripheral starts up,
there can be a rapid surge in supply demanded from one regulator
15a. This will appear as a current step on Vsup, giving a voltage
step across the on-resistance of Mc, and this may be enough to
transiently reduce Vsup below the minimum input voltage for another
regulator 15b on Vsup, or at best give a transient on this
regulator output due to its finite line regulation. Even when the
line regulation is good at d.c., it falls off with frequency, so
voltage steps on Vsup may still give transients on the regulator
outputs.
[0024] If Mc is controlled in a local regulation loop, rather than
just being turned on, this may reduce transients, but this loop
will again have finite gain and bandwidth, so there will still be
transients at some level. This would also increase the complexity
and hence cost of the circuit arrangement. Also if Mc is regulated
for example to deliver a Vsup at a fixed voltage difference below
Vbat, this voltage difference will then have to be set to a
worst-case voltage drop, which will make battery voltage headroom
under non-maximum load conditions even worse.
SUMMARY OF THE INVENTION
[0025] In general terms the present invention provides a dual
supply rail for the load regulators of a power supply circuit for a
battery powered device. One supply rail is coupled to the battery,
and the other is coupled to a non-battery source such as an
external mains regulated source and/or a bus power wire from a USB
cable or similar. The regulators are supplied from either supply
rail but through different pass or switch devices.
[0026] Preferably the regulators have dual inputs each with an
associated pass or switch device, and each for taking their input
voltage directly from one of the supply rails.
[0027] The regulators can be powered from either the battery or the
non-battery sources, but these sources are provided to the
regulators via different pass devices or transistors. This allows
these two different pass devices to have different on-resistance by
for example being of different sizes and/or types in order to
optimise cost or performance. Preferably these pass devices are
integral with the regulators themselves.
[0028] The two supply rails are effectively isolated from each
other when the battery is supplying the regulators.
[0029] The pass device will typically be a switch type of device
such as a transistor, diode, or even relay, however certain types
of regulator require a different type of pass device, and the dual
input version of this will therefore require two of these input
pass devices. For example a simple boost regulator has an inductor
as its input pass device, and a dual input version will therefore
require two input inductors.
[0030] In an embodiment the two supply rails are connected directly
to respective inputs of the regulators. As there is no switch
between the battery and the load regulators, there is no voltage
drop across such a switch device when power is supplied from the
battery. This reduced voltage drop from the battery can be used to
improve effective battery life.
[0031] The improvement in battery life can be traded off against
reduction in size of the input transistors required by the battery
inputs of the regulators and/or the cost and size of the battery.
Also the devices required for the non-battery inputs of the
dual-input regulator can typically be made smaller, since the
minimum non-battery source supply voltages are usually greater than
the minimum battery voltage, and efficiency is not so important in
the case of non-battery supply. So despite the extra transistors
required to implement the dual-input regulators, the total
transistor area will typically be reduced.
[0032] The reduction in total pass device (transistor) area not
only reduces manufacturing cost, but the reduced transistor
capacitance also reduces the power consumed by switching these
devices in switching regulators. The reduced transistor capacitance
also improves stability of linear regulators, and additionally
reduces capacitive coupling of noise on these supplies to other
circuitry on the same chip.
[0033] At the same time, the "instant-on" feature is available when
power is supplied from the non-battery (bus or external)
source.
[0034] In particular in one aspect the present invention provides a
power supply circuit for a battery powered device, the circuit
comprising: an input for receiving a first non-battery voltage
supply and coupled to a first power bus; an input for receiving a
second battery voltage supply and coupled to a second power bus; a
common load regulator having two input devices, one said input
device connected directly to the first power bus and the other said
input device connected directly to the second power bus, the common
regulator arranged to derive the regulated power supply from the
first power bus or to derive the regulated power supply from the
second power bus; and a charging circuit coupled between the first
power bus and the second power bus.
[0035] The common load regulator input devices can be optimised
depending on which power bus they are connected to. For example the
"non-battery" input devices can be made small because their
on-resistance is not critical. Whereas the "battery" input devices
can also be made smaller than known arrangements corresponding to
FIG. 3 because no additional battery switch is required. On the
other hand, the power supply circuit provides "instant-on" power
from the non-battery supply irrespective of the charge state of the
battery, unlike known arrangements corresponding to FIG. 1.
[0036] The load regulator input devices will typically be MOS based
transistors such as MOSFETS, but could be other devices, such as
diodes or bipolar transistors The power supply circuit may include
many load regulators, all supplied by the above dual power bus
arrangement.
[0037] Preferably the or each common regulator comprises switches
in order to switch between a sub-circuit for regulating the first
power bus voltage and a sub-circuit for regulating the second power
bus voltage supply.
[0038] The common load regulators may include low and non-low
dropout linear regulators, buck switching and buck-boost switching
regulators, and capacitor charge pump regulators. The use of simple
boost regulators is also possible, but generally would involve an
input inductor between each supply rail and the common internal
node. The regulators will usually be voltage regulators, but could
be constant-current regulators, or may be switchable into
constant-current modes, for example for "Hot-swap"
applications.
[0039] In particular in another aspect there is provided a power
supply circuit for a battery powered device, the circuit
comprising: an input for receiving a first non-battery voltage
supply; an input for receiving a second battery voltage supply; a
load regulator coupled to the first supply by a first switch device
and coupled to the second supply by a different second switch
device; and a charging circuit coupled between the first
non-battery voltage supply and the second battery voltage
supply.
[0040] The first and second switch devices are coupled to different
regulator inputs. Preferably the switch devices are input
transistors integral with the regulator. Preferably the devices are
MOS based transistors.
[0041] Preferably the first and second switch devices have
different on-resistances. This may be achieved with the use of
different chip area.
[0042] In general terms in another aspect there is provided a dual
input regulator having two input MOS based transistors for
receiving input voltage from two different sources; a battery
source and a non-battery source. The regulator is internally
switched in order to form an effective regulator circuit with one
of the input transistors depending on which input voltage is
used.
[0043] This allows the respective input transistors to be sized
according to their source, and can allow overall reductions in
transistor chip area as discussed above.
[0044] In particular in another aspect the present invention
provides a dual input regulator for a power supply circuit, the
regulator comprising: two input pass devices, one said pass device
for connecting directly to a first power bus and the other said
pass device connected directly to a second power bus, the regulator
arranged to derive the regulated power supply from the first power
bus or to derive the regulated power supply from the second power
bus; wherein the two pass devices are MOS-based transistors.
[0045] In one embodiment of the regulator, the two input
transistors each have first connections connected respectively to
the two voltage supply buses; the two input transistors each having
second connections connected to a regulated output, the output
being connected to an input of an error amplifier, the other input
of the error amplifier being connected to a reference voltage; the
two input transistors each having third connections switchably
connected to the output of the error amplifier.
[0046] In a further alternative, the two MOS based input or pass
devices are replaced with two input inductors, for example in the
case where the regulator is a simple boost type.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] Embodiments are described with reference to the following
drawings, by way of example only and without intending to be
limiting, in which:
[0048] FIGS. 1a and 1b show known power supply circuits for
portable battery powered devices;
[0049] FIGS. 2a to 2g show regulator circuits suitable for power
supply circuits for portable battery powered devices;
[0050] FIG. 3 shows another known power supply circuit for portable
battery powered devices;
[0051] FIG. 4 shows a power supply circuit for portable battery
powered devices according to an embodiment;
[0052] FIG. 5 shows a power supply circuit for portable battery
powered devices according to another embodiment;
[0053] FIG. 6 shows a power supply circuit for portable battery
powered devices according to another embodiment; and
[0054] FIG. 7 shows a dual input low dropout linear regulator
according to an embodiment.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0055] FIG. 4 shows a power supply circuit according to a first
embodiment and comprises two non-battery power supplies, a bus
supply 11 such as the power wires in a USB cable, and an external
supply 13 such as a pre-regulated mains source. A bus regulator 12
conditions the bus supply 11, and this and the external supply 13
are connected at a common non-battery supply node Vsup which is
coupled to a first power supply rail or bus PSR1. As with the
systems of FIGS. 1 and 3, a battery charger circuit 14 is coupled
between the non-battery common supply node Vsup and a re-chargeable
battery 10. Other components in common with FIGS. 1 and 3 are
referenced the same. The battery 10 of FIG. 4 is connected to a
second power supply rail or bus PSR2 (also Vbat). Thus two power
supply rails (PSR1 and PSR2) are provided for the load regulators
25.
[0056] The load regulators 25 each have two inputs, each with an
associated pass device Mp1 and Mp2. One such input (Mp1) is
connected directly to the first power supply rail PSR1, and the
second load regulator input (Mp2) is connected directly to the
second power supply rail PSR2. The dual input load regulators 25
are described in more detail below, however as with the regulators
of FIGS. 1, 2, and 3, these can be linear or switch mode or
capacitor charge-pump based.
[0057] Because one input (Mp2) of a regulator 25 is connected
directly to the battery 10 via the second power supply rail PSR2,
there is no voltage drop from the battery 10 to the regulator 25 as
there was in the arrangement of FIG. 3. This increases battery life
between charges as it can discharge to a lower level whilst still
supplying a minimum input voltage to the regulator 25 in order for
this to maintain its output regulation. It can also be seen that
because the first power supply rail PSR1 is independently provided
to the regulators 25, they can be instantly started when the
non-battery sources (11 and/or 13) become available, even when the
battery 10 is discharged. Thus the embodiment also overcomes the
"instant-on" problem of the arrangement of FIG. 1a and 1b.
[0058] When power is available either from the bus 11, or from the
external source 13, the load regulators 25 are driven directly from
Vsup, rather than Vbat. But when these supplies are both absent,
the load regulators 25 are supplied from Vbat, i.e. directly from
the battery 10. This removes the voltage drop caused by Mc in FIG.
3, at the expense of additional input transistors in each
regulator. However this can significantly increase battery life
which is in great demand by consumers of battery powered
devices.
[0059] Compared to the case (FIG. 3) where Mc would have dropped
150 mV, this allows an extra 150 mV in battery voltage: if this
means the battery can discharge from 4.2V to 3.45V instead of 4.2V
to 3.6V, i.e. by 750 mV rather than 600 mV, this gives an extra 25%
battery life.
[0060] Alternatively, if cost is paramount, the "battery-side"
input transistors Mp2 of the load regulators 25 can be reduced in
size to drop say 300 mV rather than 150 mV. Consider first the case
of a system with only a single regulator 15. In this case, rather
than battery switch Mc (sized for 150 mV) and "external supply
side" regulator pass device Mp (sized for 150 mV) in the circuit of
FIG. 3, we now only need a single,"battery-side", regulator pass
device Mp2 (sized for 300 mV) in the circuit of FIG. 4. The
on-voltage at the rated current is defined by the on-resistance of
the MOS, and this is approximately inversely proportional to area
of the MOS. For a given on-resistances (R) for each of Mc and Mp to
achieve a 150 mV voltage drop, and thus a total drop of 300 mV from
the battery 10, the on-resistance of Mp2 can be double Mc or Mp
(i.e. 2R) and still achieve double the volt drop (300 mV). So
instead of two transistors Mp and Mc, each of resistance R, and
corresponding area A, we now need only one MOS (Mp2) of resistance
2R and corresponding area A/2, and hence only a quarter of the area
in total. The same argument applies to the case of multiple
regulators, assuming Mc is sized to drop 150 mV for the total
current of all the regulators and the input device Mp or Mp2 of
each regulator is sized to drop 150 mV or 300 mV respectively at
the peak current of each regulator.
[0061] In addition, whilst we require the transistors Mp1 connected
to the first power supply rail PSR1 (Vsup), these will only have to
cope with a minimum Vsup of say 4.35V (4.5V bus supply 11, less 150
mV for Ma). This means that these transistors can be designed for a
1.05V drop-out rather than 150 mV, so can be made much smaller, and
so the overall chip area occupied by the transistors Mp1 is not
significant compared to the potential saving in area from removing
Mc and shrinking Mp to serve as Mp2.
[0062] The efficiency of linear regulators will be unaffected by
this sizing, since power dissipated is the product of the load
current and the input-output voltage differential. The efficiency
of any switching regulators when driven from PSR1 will however be
degraded by increasing the respective Mp1 switch resistances.
Reduced efficiency per se is not a major concern when using
external supplies, but the resulting on-chip power dissipation may
be, to avoid extra heat-sinking, or having to restrict the charger
current during times of heavy switching-regulator current load, so
this will place a lower limit on the size of Mp1. Even so, a
substantial overall saving in area is possible.
[0063] The reduction in total pass device (transistor) area not
only reduces manufacturing cost, but the reduced capacitance also
reduces the power consumed by switching these devices in switching
regulators, eases the stability of linear regulators, and also
reduces capacitive coupling of noise on these supplies to other
circuitry on the same chip.
[0064] In practice the design will be a trade-off between the
potential reduction in die size and cost from reducing the total
MOS area, and reducing the minimum battery voltage to prolong
active battery life without increasing battery size and cost.
[0065] As a side benefit, as regards practical chip layout, it is
easier to layout multiple smaller transistors rather than fewer big
transistors, so the overall chip is much easier to layout in
practice.
[0066] Regulator supply cross-talk when battery-fed is also
reduced. This is where a peripheral on one load regulator 25a
starts or suddenly draws a lot of power, which causes a dip in the
voltage (Vbat) supplied to the other regulators 25b and hence may
affect their outputs to other peripherals. In the arrangement of
FIG. 4, as there is now no common switch impedance (Mc) in the path
(only a modest non-zero output impedance of the battery), the path
impedance is significantly reduced therefore reducing the potential
voltage dip under these transient conditions.
[0067] FIG. 5 shows a modification to the circuit of FIG. 4 in
which a DC-DC converter 30 is coupled between the battery coupled
power supply rail PSR2 and the bus supply 11. In some recent bus
standards, such as USB On-The-Go, the battery-powered peripheral
may be expected to supply power to devices attached to the (USB)
bus 11. Generally the voltage required (e.g. 5V nominal over USB
downstream) is greater than the battery voltage (e.g. 3.0V to
4.2V), so a DC-DC converter 30 is required to up-step the battery
voltage. In this mode, the path from the bus 11 to Vsup is turned
off.
[0068] If the peripheral (i.e. devices coupled to the output of the
load regulators 25) is powered from the alternative supply 13, it
would be desirable to use this power, rather than discharge the
battery 10, to power the bus 11 e.g. for USB downstream. In the
circuit of FIG. 5, this would involve passing current through the
charger regulator, effectively re-charging the battery to make up
for the current drain through the bus. This is obviously
inefficient, as voltage will be dropped from the say 5V of the
supply 13, down to 4.2V or less of the battery, and then converted
back up to 5V or so by the DC-DC converter. It would also increase
the heat dissipated inside the package and therefore enlarge the
possibility of thermal shutdown.
[0069] FIG. 6 illustrates a preferable solution in which another
path is added, involving another pass device Mr and associated
control circuitry 35 to provide a more direct path from the supply
13 to the bus 11 as shown, avoiding the efficiency and thermal
problems. This also avoids distortion of the battery charging
current as monitored by charger control 14 by the current that
would otherwise have been taken by DC-DC converter 30, in the
scenario where the supply 13 is supplying power to the system and
the USB bus is requiring power.
[0070] Referring now to FIG. 7, a dual input low dropout linear
regulator 25x is shown. The regulator 25 comprises two pass devices
such as MOSFETs Mp1 and Mp2, an error op amp 26, and two switches
27a and 27b. The switches 27 are configured by control inputs (not
shown) to arrange the regulator to accept either Vbat (from the
second power supply rail PSR2 of FIG. 4) or Vsup (from the first
power supply rail PSR1 of FIG. 4) as the regulator's input.
[0071] For example it can be seen in the configuration illustrated
that Mp1 (coupled to PSR1) is effectively switched into the
regulator circuitry whereas Mp2 (coupled to PSR2) is switched out,
there being no connection between it and the error amplifier
output. The output voltage Vout is compared to the desired voltage
Vref by the error amplifier 26. If PSR1 is to act as supply, the
output of the amplifier 26 is steered to the gate of PMOS MP1. If
PSR2 is to act as supply, then the opposite connections are
made.
[0072] This dual input regulator corresponds to the linear
regulator in FIG. 2a, and in which the common node Vx of the two
pass devices is also the regulator output node. Other dual-input
regulators may be designed in a similar fashion, with the pass
device Mp (or L) shown in FIGS. 2b to 2g replaced by a pair of
devices Mp1, Mp2. each connected to a respective supply rail and
the common node Vx. In the case of the boost capacitor charge pump
of FIG. 2f, there are actually two pass devices Mp and Mp', each to
be replaced by a pair of devices Mp1, Mp2 and Mp1', Mp2' to the
respective common internal nodes Vx, Vx'. In the case of the boost
switching regulator of FIG. 2e, the dual-input regulator would have
a pair of input inductors L connected between respective supply
rails and the common node Vx.
[0073] Pass devices other than MOS based transistors (or inductors
L) could alternatively be used, such as bipolar transistors or
diodes. In some cases or modes of operation the feedback circuitry
may be omitted (such as an "open-loop" capacitor charge pump
voltage doubler based on FIG. 2f) or disabled (perhaps to turn the
pass device hard on under low-voltage conditions).
[0074] The skilled person will appreciate that the various
embodiments and specific features described with respect to them
could be freely combined with the other embodiments or their
specifically described features in general accordance with the
above teaching. The skilled person will also recognise that various
alterations and modifications can be made to specific examples
described without departing from the scope of the appended
claims.
* * * * *